Oops... I never noticed the doors closed until you took the book (I Rush Attacked him, so was close to the blood alter, and my focus stayed there until I was done looting.)
At level 15 Amdusias is not very difficult, especially if you get in close and switch him away from using a bow and the exploding arrow skill.
BTW, you may be long past this point by now, but your first encounter on sentinel island you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.